Check the /etc/apache2/conf.d/squirrelmail.conf file, if it contains a open besdire path restrictions, then ensure that the location were there pear libraries are installed on your server are included in that path.

Check the /etc/apache2/conf.d/squirrelmail.conf file, if it contains a open besdire path restrictions, then ensure that the location were there pear libraries are installed on your server are included in that path.